This application relates to an apparatus and method for optical inspection of semiconductor wafers. In particular, the apparatus and method of the present invention provide high-throughput inspection of wafers using a plurality of independent good sensitivity, low-cost inspection devices whose packaging and form factor allow operation in parallel. The present invention may be applied to brightfield and/or darkfield inspection systems.
Optical inspection of semiconductor wafers has become a standard step in the production of semiconductors. Wafers are illuminated with light emanating from a controlled illuminator, and an image of the surface is constructed based on the portion of the light reflected or otherwise directed to a light sensor. The image is processed to isolate defects from valid structures.
The sensitivity of optical inspection systems to small sized defects on a patterned wafer surface is determined by the ability of the system to discriminate between a defect signal and a valid structure signal. In brightfield inspection systems, sensors register light reflected from the wafer surface typically achieving pixel-to-defect ratio sensitivities of 1:1 or 2:1. In darkfield systems, sensors register the scattered and diffracted light, the light which deviates from a perfect reflection. In contrast to brightfield systems, darkfield systems can easily achieve a 10:1 or 20:1 pixel-to-defect ratio for certain common defects.
Optical inspection systems are either imaging or non-imaging. In imaging systems, a lens captures light reflected from an area on the wafer surface and preserves the spatial information encoded in that light (e.g., a spatial distribution of light intensity). Sensors are typically arrays of light-sensitive detectors such as charge-coupled device (CCD) "cameras" or, more recently, CMOS photodiode or photogate cameras.
In contrast, in non-imaging systems the light from the illuminator is concentrated on a small area (ideally a very small point) on the wafer's surface. A sensor--for example a photomultiplier tube, photodiode, or avalanche photodiode--detects scattered, or diffracted light, and produces a signal proportional to the integrated light intensity.
Others have concentrated on attempting to maximize sensitivity in order to detect the defects of the smallest size. Consequently, expensive special-purpose optical, mechanical, electronic, and computer processing systems have been employed in state-of-the-art tools. For example, these systems may allow adjustments in magnification levels, illumination angles, and polarization, each of which further increases system complexity and cost. The components in each sensing subsystem of the present invention are limited to performing only one type of inspection, further reducing cost by eliminating complexity. The present invention's individual subsystems emphasize compactness and low-cost over sensitivity, providing a system with medium sensitivity and high levels of throughput. Increasing the throughput of such systems stresses the design of these components and further increases the cost.
Some applications do not require the high level of sensitivity produced by such systems. In equipment monitoring, for example, a statistical process may be used to indicate whether the equipment used during a manufacturing step is functioning correctly. The sensitivity, in this case, need only be high enough to detect when the manufacturing equipment is causing a statistically significant excess number of defects (excursion). Since this type of application is most useful if repeated often (after every manufacturing step, for example), fast examination speed is important to keep up with the manufacturing process.
What is needed therefore is a fast and relatively inexpensive tool for monitoring the condition of wafers at various points in the integrated circuit manufacturing process.